CIE iGCSE Co-Ordinated Science B5. Enzymes Exam Style Questions Paper 3
Question
- have the widest pH range of activity …………………….. and ……………………..
- is active at pH 5.5 …………………………………………
- is most active in alkaline conditions. …………………………………………
hormones proteins solvents
oesophagus pancreas small intestine
mechanical digestion …………………..
egestion ……………………
ingestion …………………….
Most-appropriate topic codes (Cambridge IGCSE Co-ordinated Sciences 0654):
• Topic B5.1 — Enzymes (Part (a), (b))
• Topic B7.2 — Digestive system (Part (c), (d))
▶️ Answer/Explanation
(a)(i) widest range: A and C; active at pH 5.5: A; most active in alkaline: C
Enzyme A (4.5–8.5) and enzyme C (7.0–11.0) both span 4.0 pH units, the widest ranges in the table.
Only enzyme A’s range (4.5–8.5) includes pH 5.5.
Enzyme C has the highest “most active” pH value (9.0), which is alkaline.
(a)(ii) Temperature (or substrate concentration)
Enzyme activity is also affected by temperature, since kinetic energy and denaturation depend on it.
(b) catalysts and proteins
Enzymes are biological catalysts that speed up reactions without being used up.
Structurally, enzymes are made of protein.
(c) mechanical digestion: mouth; egestion: anus; ingestion: mouth
Mechanical digestion (chewing) occurs in the mouth, physically breaking food into smaller pieces.
Egestion (removal of undigested waste as faeces) occurs at the anus.
Ingestion (taking food into the body) occurs at the mouth.
(d) Both involve the movement of digested food / nutrient molecules
Absorption is the movement of digested nutrients from the intestine into the blood, while assimilation is their uptake and use by cells — both processes involve the movement of soluble nutrient molecules.
Question
- protein
- glycogen
Most-appropriate topic codes (Cambridge IGCSE Co-ordinated Sciences 0654):
• Topic B4.1 — Biological molecules (Part (a)(i), (a)(ii) & (a)(iii))
• Topic B5.1 — Enzymes (Part (b)(i) & (b)(ii))
▶️ Answer/Explanation
(a)(i)
Protein: carbon, hydrogen, oxygen, nitrogen
Glycogen: carbon, hydrogen, oxygen
Proteins are built from amino acid monomers, which all contain C, H, O, and N in their basic structure. Glycogen is a polysaccharide made from glucose monomers, which contain only carbon, hydrogen, and oxygen — the same three elements found in all carbohydrates.
(a)(ii)
glycerol and fatty acids
A fat molecule (triglyceride) is formed when one glycerol molecule bonds to three fatty acid molecules through condensation reactions. Each bond formed is an ester linkage, releasing a water molecule, and the three fatty acids attached may be the same or different depending on the type of fat.
(a)(iii)
Test: Biuret test — add dilute copper(II) sulfate solution followed by sodium hydroxide solution.
Positive result: purple / violet colour
The Biuret reagent reacts with peptide bonds present in all proteins and polypeptides, producing the characteristic purple colour. A negative result (no protein present) leaves the solution blue, which is the colour of the copper(II) sulfate alone.
(b)(i)
Enzymes are proteins that are involved in all metabolic reactions, where they function as biological catalysts.
Enzymes speed up metabolic reactions by lowering the activation energy required, without being consumed or permanently altered in the process. Their specificity comes from the complementary shape of their active site, which fits only particular substrate molecules.
(b)(ii)
Graph A $(\checkmark)$
As temperature rises, increased kinetic energy leads to more frequent enzyme–substrate collisions, raising reaction rate up to the optimum temperature. Beyond the optimum, the enzyme’s active site is distorted as hydrogen bonds break down in a process called denaturation, causing an irreversible loss of activity and a sharp decline in reaction rate.